In recent years, improvement of the fuel efficiency of automobiles is required to control CO2 emission from the viewpoint of global environment conservation. In addition to this, an improvement in safety, such as the crashworthiness of automobile bodies is also required to guarantee occupant safety when an automobile crush occurs. Therefore, the progress is made in the weight reduction and strengthening of automobile bodies.
It is said that, to realize the weight reduction and strengthening of automobile bodies at the same time, it is effective to reduce the weight of automobile bodies by the high strengthening of the steel sheets and by decreasing the thickness of the steel sheets as long as there is no problem regarding stiffness, and high strength steel sheets are actively used for the parts of automobiles nowadays.
On the other hand, since many automobile parts made from a steel sheet are formed by press forming, a steel sheet used for automobiles is required to have good press formability. However, since a high strength steel sheet is much poorer in terms of formability, in particular deep drawability, than an ordinary mild steel sheet, there is an increased desire for a steel sheet having a tensile strength TS of 440 MPa or more and good deep drawability to reduce the weight of automobile bodies. Specifically, in terms of Lankford value (hereinafter, referred to as r value) which is the evaluation indicator of deep drawability, a steel sheet having an average r value of 1.20 or more is required.
In addition, since a high strength steel sheet contains various alloy elements in large amounts to realize high strengthening, the kinds and amounts of precipitates present in the steel, widely vary due to change in manufacturing conditions, which results in a tendency for change in mechanical properties in a coil to increase in particular in the longitudinal direction of the coil. In the case where change in mechanical properties is large, it is difficult to stably perform press forming in a continuous pressing line for automobile bodies, which results in a significant decrease in operability. Therefore, the uniformity of the mechanical property in a coil is strongly required.
To solve the problems described above, as a means of increasing strength while maintaining a high r value, for example, Japanese Examined Patent Application Publication No. 57-57945 discloses a method using an ultralow-carbon steel sheet in which chemical elements such as Si, Mn and P which are effective for solid solution strengthening are added to a base steel which is made interstitial atom free steel by adding Ti and Nb which are effective for fixing carbon and nitrogen which form solid solutions in steel.
However, to manufacture a high strength steel sheet having a tensile strength of 440 MPa or more by using the method described above in which chemical elements which are effective for solid solution strengthening are added to an ultralow-carbon steel, there is an increase in the amount of added alloy elements. For example, in the case where the Si content is large, Si becomes concentrated on the surface of the steel sheet when continuous annealing is performed and combines with a small amount of water vapor in the atmosphere, Si oxides are formed on the steel sheet surface, which results in a significant decrease in phosphatability. In addition, in the case where the P content is large, P is segregated at grain boundaries, which results in a significant decrease in resistance to secondary working brittleness. In the case where the Mn content is large, there is a decrease in r value. Moreover, r value decreases remarkably with high strengthening.
In addition, there is a method of obtaining a high strength steel sheet other than a solid solution strengthening method described above. A complex phase steel sheet consisting of a soft ferrite phase and a hard martensite phase generally has good ductility, good strength-ductility balance, and low yield strength. Therefore, the steel sheet has comparatively good press formability. However, the steel sheet has a low r value and poor deep drawability. This is said to be because solid solution C (solute C), which is necessary to form a martensite phase, suppresses the formation of a {111} recrystallization texture, which is effective in increasing an r value.
As a technique to improve the r value of a complex phase steel sheet described above, for example, Japanese Examined Patent Application Publication No. 55-10650 discloses a method in which box annealing is performed at a temperature between the recrystallization temperature and the Ac3 transformation point after cold rolling has been performed, the annealed steel sheet is heated up to a temperature of 700° C. to 800° C., and the heated steel sheet is quenched and tempered. In addition, Japanese Unexamined Patent Application Publication No. 2003-64444 discloses a high strength steel sheet having a predefined C content, a microstructure including one or more of bainite, martensite and austenite phases in an amount of 3% or more in total, and an average r value of 1.3 or more. However, both the techniques disclosed in Japanese Examined Patent Application Publication No. 55-10650 and Japanese Unexamined Patent Application Publication No. 2003-64444 require annealing to increase the r value by growing a texture as a result of forming clusters and precipitates of Al and N and require heat treatment to form a desired microstructure. Further, in the techniques, box annealing is required for a long duration of one hour or more. Therefore, since box annealing is necessary, the treatment time is longer than that of continuous annealing and there is an increase in the number of processes, which results in a significant decrease in efficiency and productivity, that is, a decrease in economic efficiency from the viewpoint of manufacturing cost, and which results in many problems in the manufacturing process such as the frequent occurrence of adhesion between steel sheets, the occurrence of temper color, and a decrease in the service life of the inner cover of the furnace body.
Moreover, Japanese Unexamined Patent Application Publication No. 2002-226941 discloses a technique in which the r value of a complex phase steel sheet is improved by appropriately controlling C and V contents. In that technique, the amount of solid solution C is decreased as much as possible by precipitating C in the steel in the form of carbides containing V before recrystallization annealing is performed to increase an r value, and then the carbides containing V are dissolved by heating the steel sheet under the conditions for forming an α-γ dual phase to concentrate C in the γ phase, which results in the formation of a martensite phase in a cooling process afterwards.
However, in the method in which carbides containing V is dissolved when annealing is performed under the conditions for a dual phase, since there is a concern that mechanical properties may vary due to variation in dissolving speed, it is necessary to control an annealing temperature and an annealing time with a high degree of accuracy, which results in a problem in manufacturing stability in practice.
In addition, Japanese Unexamined Patent Application Publication No. 2005-120467 discloses a technique in which an increase in r value and formation of a complex phase are realized at the same time by controlling a chemical composition to contain, by mass %, the C: 0.010% to 0.050% and the Nb content and the C content to satisfy the relationship 0.2≦(Nb/93)/(C/12)≦0.7. An increase in r value is intended in this technique by retaining solid solution C, which is necessary to form a martensite phase after annealing, at the stage of hot rolled steel sheet and by utilizing an effect of grain refinement of the microstructure of a hot rolled steel sheet by adding Nb and an effect of decreasing the amount of solid solution C due to the precipitation of NbC.
However, in the technique disclosed in Japanese Unexamined Patent Application Publication No. 2005-120467, since an increase in r value is intended by utilizing an effect of grain refinement of the microstructure of a hot rolled steel sheet by adding Nb and an effect of decreasing the amount of solid solution C due to the precipitation of NbC, there are problems in that Nb is very expensive and in that Nb significantly delays the recrystallization of an austenite phase, which results in an increase in the rolling load at hot rolling. In addition, Nb which is precipitated in a hot rolled steel sheet causes an increase in deformation resistance when cold rolling is performed, which results in an increased risk that troubles may occur due to an increase in load on rolls, and which results in such problems that there are a decrease in productivity and a restriction on the available width of products. Moreover, in the case of the carbon content described above (0.010% to 0.050%), since it is difficult to control the precipitation state of NbC in a hot rolled steel coil, in particular at the front and tail edges of the coil, a cold rolled steel sheet which is made of the material having this kind of chemical composition tends to have non-uniform distribution of mechanical properties in the coil in the longitudinal direction, which results in a problem of uniformity of mechanical property in a coil.
As described above, many techniques to improvement of uniformity of mechanical property in a coil of a cold rolled steel sheet have been proposed. For example, Japanese Examined Patent Application Publication No. 61-032375 discloses a technique in which the uniformity of mechanical property in a coil is improved by adding the combination of Ti and Nb to steel having a decreased C content of 0.0070% or less and by hot rolling the steel under the condition that the coiling temperature is 620° C. or higher. In this technique, N which causes variation in mechanical properties is precipitated in the form of TiN instead of AlN before finish rolling and C is precipitated as a compound carbide in the form of (Ti, Nb)C. However, in practical operation, there is a case where a coiling temperature is 600° C. or lower or where the temperature of some part of a coil is 600° C. or lower, and in such cases there is a problem of an increase in variation in mechanical properties due to the variation of precipitation behavior in a coil. In particular, in the case where the atom ratio of Ti and Nb with respect to C is small, C is not sufficiently fixed by precipitation, and a deterioration of mechanical properties increases at the front and tail edges of a coil which are comparatively prone to be cooled.
In addition, Japanese Unexamined Patent Application Publication No. 2000-303141 discloses a technique in which dependence of mechanical properties such as strength and elongation on a coiling temperature is decreased by controlling a chemical composition such that the C content is more than 0.0050% and 0.010% or less and (Nb %×12)/(C %×93) is 1.6 to 2.4. However, that technique is intended for ferrite single phase steel which is made using IF steel (Interstitial Free steel) as base steel, which is ultralow-carbon steel, and there is no mention of a high strength steel sheet having a tensile strength of 440 MPa or more.
As described above, in the case of a method of high strengthening of a steel sheet by solid solution strengthening which has been investigated in order to increase the strength of a mild steel sheet having good deep drawability, it is necessary to add large amounts of alloy elements, which causes problems, for example, regarding cost and phosphatability and regarding increasing r value.
In addition, in the case of the methods utilizing transformation strengthening, it is necessary to perform annealing twice and to use a high speed cooling apparatus, and therefore there are problems in manufacturing processes. Although a method utilizing V and C is also disclosed, there is a concern that mechanical properties may vary due to variation in the dissolving speeds of V and C, and it is necessary to control an annealing temperature and an annealing time with a high degree of accuracy, which results in a problem in manufacturing stability in practice.
Moreover, although a technique in which an increase in the r value of a dual steel sheet is intended by utilizing an effect of grain refinement of the microstructure of a hot rolled steel sheet by adding Nb and an effect of decreasing the amount of solid solution C due to the precipitation of NbC is disclosed, there are problems in that Nb is very expensive and Nb significantly delays the recrystallization of an austenite phase, which results in an increase in the rolling load at hot rolling. Moreover, NbC which is precipitated in a hot rolled steel sheet causes an increase in deformation resistance when cold rolling is performed, which results in difficulty in stable manufacturing in practice. Moreover, regarding uniformity of mechanical property in a coil, it is difficult to control the precipitation state of NbC in a hot rolled steel coil, in particular at the front and tail edges of the coil, which results in non-uniform distribution of mechanical properties in the longitudinal direction in a coil.
It could therefore be helpful to provide a high strength cold rolled steel sheet with excellent deep drawability and uniformity of mechanical property in a coil which is suitably used for the inner and outer panels of automobile bodies and a method for manufacturing the steel sheet.